1. Field of the Invention
The present invention relates to a sheet conveyance apparatus that can correct a skew of a sheet and an image forming apparatus including the same.
2. Description of the Related Art
An electrophotographic copying machine forms a toner image and transfers the toner image onto a sheet in a transfer unit. Thereafter, the copying machine fixes the toner image onto the sheet by heating and pressurizing thereof. The conventional copying machine includes a skew correction unit for correcting a skew of the sheet at a position upstream in a conveyance direction of the transfer unit in order to transfer the toner image onto an accurate position of the sheet in the transfer unit.
Japanese Patent Application Laid-Open No. 62-244846 discusses a skew correction unit 55 including, as is illustrated in FIGS. 6A to 6C, a registration roller pair 56a, 56b (hereinafter sometimes referred to as registration roller pair 56) and a pre-registration roller pair 57a, 57b (hereinafter sometimes referred to as pre-registration roller pair 57) positioned upstream of the registration roller pair 56a, 56b in order to convey the sheet toward the registration roller pair 56a, 56b. A method for correcting the skew of the sheet by the skew correction unit 55 is described below with reference to FIGS. 6A to 6C.
FIG. 6A illustrates a state in which a sheet S is conveyed by a pre-registration roller pair 57a, 57b, and the sheet S is not yet at a nip portion of the registration roller pair 56a, 56b. At the time, the registration roller pair 56a, 56b is stopped without rotating.
The pre-registration roller pair 57a, 57b causes the sheet S to form a loop by causing the sheet S to abut against the nip portion of the registration roller pair 56a, 56b, as illustrated in FIG. 6B. As described above, since the skew correction unit 55 causes the sheet S to form the loop, in a case where the sheet S is skewed, a downstream end of the sheet S in a sheet conveyance direction is revolved along the nip portion of the registration roller pair 56a, 56b due to stiffness (i.e., elasticity) of the sheet S and, thereby, the skew of the sheet S is corrected.
As illustrated in FIG. 6C, the registration roller pair 56a, 56b rotates according to a timing at which the toner image is transferred onto the sheet S by the transfer unit. Accordingly, the sheet S is started to be conveyed while a leading edge of the sheet S is positioned along the nip portion of the registration roller pair 56a, 56b. The sheet S is conveyed to the transfer unit in a state in which the skew of the sheet S is corrected.
In a case where the conventional skew correction unit 55 executes the skew correction of a sheet having a high stiffness such as a thick paper, the following problem may occur. FIG. 7A illustrates a state before a skewed sheet S abuts against the nip portion of the registration roller pair 56a, 56b. FIG. 7A illustrates an abutting position 200 of the leading edge of the sheet S when the sheet S correctly abuts against the nip portions. In a case where the sheet S having high stiffness is conveyed in a skewed state, an abutting force concentrates at a portion (i.e., a position 201 of FIG. 7B) where the downstream end (i.e., leading edge) of the sheet S in the sheet conveyance direction firstly abuts against the nip portion of the registration roller pair 56.
Therefore, as illustrated in FIG. 7B, there is such a case that the leading edge of the sheet S is bitten into the nip portion of the registration roller pair 56 too much at a first abutting portion, and goes far beyond the position 200. When the pre-registration roller pair 57 continues to convey the sheet S while the sheet S is in the above described state, the sheet S is revolved with the leading edge of the sheet S still being bitten into the nip portion at the position 201 (i.e., the sheet S is revolved from a dotted line position to a solid line position).
Then, the downstream end of the sheet S in the sheet conveyance direction abuts against the nip portion of the roller positioned next to the roller by which the sheet S is bitten into as illustrated in FIG. 7C. At the time, since revolution of the sheet S is stopped by the roller positioned next thereto, the downstream end of the sheet S in the sheet conveyance direction cannot evenly abut against the nip portions of the registration roller pair 56.
As a result thereof, the skew correction unit 55 cannot cause the downstream end of the sheet S in the sheet conveyance direction to be positioned along the nip portions of the registration roller pair 56, and thus correction of the skew of the sheet S cannot be achieved. This problem frequently occurs when the skew correction is executed with respect to a sheet having high stiffness, e.g., a thick paper.